1. Field of the Invention
The present invention relates to gauges, and in particular to thickness gauges that are capable of determining the thickness of a coating on a substrate, regardless of the composition of the substrate and coating.
2. Discussion of related art
Ultrasound provides an ideal physical mechanism to investigate the thickness of coatings on substrata with acoustically different properties. When a coating is applied to a substrate that has acoustic properties that are different from those of the coating, an acoustic coating/substrate interface is created. At such an interface, an ultrasonic vibration will be partially reflected.
For example, an ultrasonic vibration, also known as an impulse, can be transmitted into a coating using a resonant piezo element transducer. The same transducer can also be setup to "listen" for echoes created when the impulse reflects from the interface of the coating and substrate back to the transducer. The output of the transducer can be recorded for a known period after the impulse has been transmitted. This period is defined as an echo window. The echo window is defined to overlap with the time of expected echoes of interest.
By analyzing the echo recorded during the echo window, it is possible to determine the location of the interface between the coating and the substrate. The thickness of the coating can be determined if the velocity of sound within the coating material and the time of the interface echo are known. In other words, the thickness of the coating can be determined by multiplying the velocity of the vibration through the coating material times the time for the vibration to enter the coating, reflect off the interface, and exit the coating, and dividing that product by two. EQU Thickness=(Velocity.times.Time)/2
The resolution of the derived thickness is limited by the temporal resolution of the sampled echo. Improvements in the resolution of the sampled echo will directly improve the resolution of the derived thickness.
Ultrasonic coating thickness gauges used to measure coatings on nonferrous, nonconductive substrata have existed for some time. The gauges fall into three broad categories: Real Time Echo Analyzers, Real Time Echo Capture/Digital Analyzers, and Analog/Digital Hybrid Flaw Analyzers. A brief description of each of the types of gauges follows: